Traditional weight lifting exercise machines typically couple a stack of iron weights through a series of pullerys and levers to hand grips or other types of user interfaces that are utilized by the user to lift the weight stack. To vary the force opposing the user in such weight lifing machines, the user is typically required to change the position of a mechanical locking pin and physically add or remove weights from the stack. Because it is relatively time consuming and inconvenient to change the exercise force level between lifts in such machines, traditional weight lifting machines have not been used to implement exercise programs in which the force level is varied from lift to lift.
Electronically controlled exercise machines are known in which a hydraulic cylinder is used to provide a resistance opposing the movements of user, the cylinder, for example, being controlled by a computer through a stepper motor. An example of such a machine is disclosed in U.S. Pat. No. 4,544,154. Although this type of exercise machine is more flexible than traditional weight lifting machines, they tend to be very costly to manufacture and maintain. They are also limited in that they only provide a resistance to the user's motions and as such cannot be used to accurately simulate a weight machine nor to implement a heavy negative type program.
Also, attempts have been made to create exercise machines employing motors to provide a force to oppose the user. An example of one such machine is disclosed in UK Patent Application GB No. 2 157 578A. However, these machines generally have problems with stability and do not adequately simulate the feel of a weight stack to the user. A description of an apparatus that overcomes certain of the stability problems is provided in the co-pending U.S. patent application entitled "Motor Control Circuit For A Simulated Weight Stack," Ser. No. 107,970, filed Oct. 13, 1987 and assigned to the assignee of this application.
Electric motors have been used in exercise and rehabilitation machines such as the apparatus disclosed in U.S. Pat. No. 4,691,694. In this apparatus a closed loop servo system using a user force signal and a speed signal drives a fixture. The limits of motion of the fixture can be manually set by the user. However, these machines are not designed to simulate a weight stack.
An example of the implementation of a heavy negative force is provided in U.S. Pat. No. 4,563,003 where a motor driven rod is used in a conventional weight stack machine to increase the user opposition force during a downward stroke.